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Vitamin A

March 21, 2017

Vitamin A

Other name(s):

b-carotene, beta-carotene, retinol, vitamin A-1

General description

Vitamin A is also called retinol. It was the first substance isolated in the group called vitamins. It is one of the four fat-soluble vitamins. It’s found in animal products. Fat-soluble vitamins are stored in the body. They can build up to toxic levels if you take too much of them.

Precursors of vitamin A exist in plants. They’re called carotenoids. These are fat soluble but non-toxic, even in large quantities. The best known carotenoid is beta-carotene. Both retinoids and carotenoids are good antioxidants.

Vitamin A plays a role in the reproductive process. It’s involved in the development of sperm (spermatogenesis). It’s also involved in the development of the fetus. But excessive doses of vitamin A and synthetic retinols may lead to fetal malformation. It may also lead to congenital issues. Vitamin A seems to guide the developing structures in the fetus. It also helps the placenta form during pregnancy.

Vitamin A is an important factor in growth throughout life. Vitamin A is involved in the development and maintenance of epithelial tissues. These include mucous membranes, the lining of the gastrointestinal tract, lungs, bladder, urinary tract, vagina, cornea, and skin. Vitamin A also helps the growth of bones and teeth.

Vitamin A prevents drying of the skin. This may protect the body from infectious diseases. It’s also involved in maintaining the immune system (T-lymphocytes and phagocytosis).

Vitamin A is also needed for night vision. Retinol (a vitamin A metabolite) combines with opsin (a pigment in the retina of the eye) to form rhodopsin. This is a chemical involved in night vision.

Medically valid uses

Vitamin A plays an important role in the reproductive process, growth, and development. It also keeps eyes and skin healthy, and acts as an antioxidant.

Vitamin A supplements may reduce the risk for certain types of cancer. Vitamin A influences cell development. It also increases the activity of immune-system cells. This could make it valuable in the fight against cancer, especially skin, lung, bladder, and breast cancer.

Unsubstantiated claims

Please note that this section reports on claims that have not yet been substantiated through studies.

Vitamin A may prevent some types of cancer and acne. It may also help treat psoriasis. It’s also claimed to help treat dry or wrinkled skin. It may also protect against the effects of pollution and prevent respiratory tract infections. Vitamin A may aid in healing sunburn. It’s also been used to treat kidney stones, inflammatory bowel disease, and deafness.

Vitamin A has been called the anti-infection vitamin because of its role in helping the body combat bacterial, parasitic, and viral infections.

Recommended intake

Vitamin A is measured in Retinol Activity Equivalents (RAE) to account for the different bioactivities of retinol and provitamin A carotenoids. The RDA is the Recommended Dietary Allowance.

Vitamin A is listed on food and supplement labels in international units (IUs). But scientists rarely use this measure. Conversion rates between mcg RAE and IU are as follows:

1 IU retinol = 0.3 mcg RAE

1 IU beta-carotene from dietary supplements = 0.15 mcg RAE

1 IU beta-carotene from food = 0.05 mcg RAE

1 IU alpha-carotene or beta-cryptoxanthin = 0.025 mcg RAE

Group

RDA (RAE)

Infants (0–6 months)

400 mcg*

Infants (7–12 months)

500 mcg*

Children (1–3 years)

300 mcg

Children (4–8 years)

400 mcg

Children (9–13 years)

600 mcg

Males (14 years and older)

900 mcg

Females (14 years and older)

700 mcg

Pregnancy (14–18 years)

750 mcg

Pregnancy (19 years and older)

770 mcg

Breastfeeding (14–18 years)

1,200 mcg

Breastfeeding (19 years and older)

1,300 mcg

*Adequate Intake (AI). This is based on the average intake in healthy, breastfed infants.

Smaller people need less vitamin A than larger people do. In most cases, women need less than men, except during pregnancy and while breastfeeding.

Food source

Nutrient content per 100 grams

Cod liver oil

85,000 IU

Beef liver

20,000 IU

Carrots

11,000 IU

Sweet potatoes

8,800 IU

Parsley

8,500 IU

Spinach

8,100 IU

Cantaloupes

3,400 IU

Apricots

2,700 IU

Vitamin A is stable at room temperature. It doesn’t need to be refrigerated. Cooking, freezing, or canning vegetables and fruits doesn’t destroy much vitamin A. But you shouldn’t freeze vitamin A tablets and capsules. Vitamin A is also stable in light.

A poor diet with inadequate intake of vitamin A can increase your need for vitamin A. So can diets containing large amounts of snack foods containing the fat substitute Olestra.

Malabsorption syndromes marked by excess fat in the stool (steatorrhea) may deplete all four fat-soluble vitamins: A, D, E, and K. There are many common malabsorption syndromes. These include lactose intolerance, tropical and non-tropical sprue, celiac disease, and cystic fibrosis. They also include ulcerative colitis, Crohn's disease, and steatorrhea caused by removal of all or part of the pancreas (pancreatectomy).

Other reasons for increased need for vitamin A include prolonged fever, hyperthermia, and infection. They also include chronic use of mineral oil, diabetes, or hyperthyroidism.

An early sign of vitamin A deficiency is night blindness. This condition may start with decreased ability to see at night or in the dark. The time required for eyes to adjust to the dark increases until you lose the ability to see at night.

As vitamin A deficiency continues, xerophthalmia (conjunctival dryness) may develop. This starts with a dry feeling in your eyes. The eyelid linings also become dry (xerosis conjunctivae) and rough. As the issue continues, the cornea dries out (xerosis cornea) and becomes wrinkled and cloudy. Scarring then occurs with irreversible changes. This leads to blindness.

Skin changes are another sign of too little vitamin A. The skin becomes dry and rough (follicular hyperkeratosis). This is very noticeable over the shoulders, buttocks, and the extensor surfaces (the opposite side of a joint) of the arms and legs. Little bumps may show up around the base of each hair. This causes a sandpaper-like texture to the skin.

Mucous membranes may also change. This may affect the lining of the urinary tract. This may cause burning and bleeding with urination. The lining of the vagina may also get dry and inflamed.

Retinol is used to treat vitamin A deficiency. In many undeveloped countries, vitamin A deficiency is common. Because vitamin A can be stored in the body, large doses can be given to children (and some adults) only two or three times a year. This is done to prevent xerophthalmia, a condition that leads to blindness. Vitamin A deficiency is rare in the U.S. When it occurs, it’s usually due to malabsorption caused by other diseases.

Side effects, toxicity, and interactions

Vitamin A in the form of beta-carotene is considered safe. It doesn’t appear to be toxic in large doses. However, high doses over a long period of time can lead to carotenemia. In this condition, your skin becomes yellowish orange.

Vitamin A overdose in the form of retinoids (from animal sources) can be toxic. The conditions of overdose, or hypervitaminosis, are divided into two groups: acute and chronic. These are further separated into infant and adult.

Children are more sensitive than adults to overdoses of vitamin A. The symptoms of an acute overdose in an infant or child include:

Nausea and vomiting

Drowsiness

Excessive sleepiness

Bulging soft spot (fontanel) on the top of the infant's head

Pseudotumor cerebri. This condition increases pressure around the brain, bulging of the optic disc in the back of the eye, paralysis, or change in function of some of the cranial nerves. This is seen after the soft spot has closed over and the sutures fused.

The symptoms of a chronic overdose in an infant or child include the following:

Loss of appetite

Itching

Slowed weight gain

Irritability

Hair loss (alopecia)

Bone tenderness

Swelling

Skin changes. These include dryness, roughness, and cracks in the corners of your mouth.

In adults, symptoms of an acute overdose may include:

Headache

Nausea and vomiting

Irritability

Seizures

Double vision

The signs of a chronic overdose in adults include the following:

Skin changes. These include dryness, roughness, and fissuring of the lips and corners of the mouth.

Hair loss

Loss of appetite

Increased pressure around the brain. This may cause symptoms that look like a brain tumor.

You shouldn’t take vitamin A supplements if you’re allergic to vitamin A.

You shouldn’t take very high doses of vitamin A during pregnancy. This is because high doses may cause malformations in your forming fetus.

Mineral oil, cholestyramine, and foods containing Olestra may interfere with the absorption of vitamin A. Orlistat, a medicine for weight loss, has been shown to decrease absorption of beta-carotene and vitamin E. Whether Orlistat has the same effect on vitamin A is unknown.

You shouldn’t take vitamin A supplements if you’re taking isotretinoin, acitretin, or etretinate.

If you’re using topical ointments that are in the retinoid family, ask your healthcare provider if it’s safe for you to take vitamin A supplements.

Taking tetracycline with high doses of vitamin A may cause benign intracranial hypertension. You shouldn’t take this medicine with vitamin A.

Oral birth control pills increase the levels of vitamin A in your body. For this reason, you may not need vitamin A supplements.